This invention relates to a process for operating a fuel-burning engine which uses a conventional fuel containing an alternative fuel such as alcohol, the fuel-alcohol ratio being measured for the purpose of adjusting the quantity of fuel to be added at any time.
Given that the fossil energy reserves in the long run will be available only in limited quantities, in particular fuels obtained from crude oil, and given the increasing environmental protection requirements, alternative fuels, in particular methyl or ethyl alcohol, are being added to these fuels in increasing amounts. It should therefore be possible to arbitrarily refuel both with pure fuels and mixed fuels. With higher alcohol content it is necessary to know the blending ratio in order to attain optimal performance from the fuel-burning engine and to facilitate a precise proportioning of fuel adjusted to the operating conditions. The continuous determination of the alcohol content in the fuel, continuously fed to the fuel-burning engine in operation, presents special problems for automobile engines in which any possible blend can be attained by any arbitrary refueling of types of fuel.
The known optical processes are unsuitable for this purpose since they often utilize interface effects to determine the refraction index from which then the alcohol content can be inferred. Besides the difficulty of utilization in automobile engines, one drawback of such process is that the mix to be observed with a measurement must have a higher homogeneity, which must also be present especially at the boundary layer. The required precision has not been achieved with this process.
In the publication "Proceedings of the Fourth International Symposium on Alcohol Fuels Technology," Sao Paulo, Brazil, of October 5, 1980, the possibility of determining the alcohol content in fuels by means of dielectric measurements is described. However, due to the influences of temperature and conductance (induced by water content or other pollutants in the fuel), the process was rejected, since a reliable measurement suitable for fuel-burning engines could not be conducted.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for operating a fuel-burning engine of the aforementioned type that from the standpoint of equipment and circuits can be effected in a simple manner with high operating reliability and efficiency.
The invention provides that in a process of the aforementioned type the measurement of the alcohol content in the fuel fed in serves to pre-control the quantity of fuel fed in, while the air ratio is precision controlled by means of a known lambda control. Moreover, a better start and warm-up performance can be achieved for the fuel-burning engine, since the lambda control can start at higher exhaust temperatures. Thus, for example, an adaptive control can be realized in that the measuring cell or the measuring circuit and/or the evaluation circuit is adjusted by means of the correcting quantity of the lambda control.
According to one preferred embodiment the alcohol content of the fuel is determined in a shared circuit by measuring the capacitance and the conductance of a quantity of fuel accommodated in a measuring cell.
Furthermore, the shared circuit is oscillatory and its output frequency is evaluated as the measure of capacitance, whereby the quantity of fuel in the measuring cell or a portion thereof forms the dielectric of the capacitive part of the circuit.
The shared circuit can be a flip-flop circuit in which the quantity of fuel in the measuring cell or a portion thereof forms the dielectric of the frequency determining capacitor. The frequency of the flip-flop circuit can be evaluated to determine the capacitance of the circuit.
Special advantages can be achieved if the duty factor or one of the two switching conditions of the flip-flop circuit is evaluated to determine the conductance of the circuit.
Another preferred embodiment provides that prior to evaluating the measuring signal, a pre-processing is conducted in that the signal is converted by means of integration into a voltage corresponding to the duty factor.
It can also be expedient that prior to evaluating the measuring signal a pre-processing is conducted in that during the phase with high potential a counter counts up an external, higher frequency signal and during the phase with low potential it counts down such that the count at the end of the period is a measure for the duty factor.
The measured values can be scanned from the at least partially conductive wall of the measuring cell, which represents the first electrode, and from an at least partially conductive flow body, which represents the second electrode.
From the standpoint of construction and circuit technology it is expedient if the shared circuit and/or the evaluation of the measuring signals is/are integrated with a program and circuit into the fuel proportioning system.
Another preferred embodiment provides that the measuring cell be used as the frequency-determining member in a generator, whose frequency is changed by means of the parallel circuit of a known capacitor, whereby the two different frequencies are evaluated to determine the capacitance by means of computer or circuit technology. The frequency can also be changed by a switchable delay.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.